In the last years, an increasing extension of communication networks, e.g. of wire based communication networks, such as the Integrated Services Digital Network (ISDN), or wireless communication networks, such as the cdma2000 (code division multiple access) system, cellular 3rd generation (3G) communication networks like the Universal Mobile Telecommunications System (UMTS), cellular 2nd generation (2G) communication networks like the Global System for Mobile communications (GSM), the General Packet Radio System (GPRS), the Enhanced Data Rates for Global Evolutions (EDGE), or other wireless communication system, such as the Wireless Local Area Network (WLAN) or Worldwide Interoperability for Microwave Access (WiMax), took place all over the world. Various organizations, such as the 3rd Generation Partnership Project (3GPP), Telecoms & Internet converged Services & Protocols for Advanced Networks (TISPAN), the International Telecommunication Union (ITU), 3rd Generation Partnership Project 2 (3GPP2), Internet Engineering Task Force (IETF), the IEEE (Institute of Electrical and Electronics Engineers), the WiMax Forum and the like are working on standards for telecommunication network and access environments.
Within the IP (Internet Protocol) Multimedia Subsystem (IMS) as defined by 3rd Generation Partnership Project (3GPP) Session Initiation Protocol (SIP) defined by Internet Engineering Task Force (IETF) is used for controlling communication. SIP is an application-layer control protocol for creating, modifying, and terminating sessions with one or more participants. These sessions may include Internet multimedia conferences, Internet telephone calls, and multimedia distribution. Members in a session can communicate via multicast or via a mesh of unicast relations, or a combination of these. Session Description Protocol (SDP) is a protocol which conveys information about media streams in multimedia sessions to allow the recipients of a session description to participate in the session. The SDP offers and answers can be carried in SIP messages. Diameter protocol has been defined by IETF and is intended to provide an Authentication, Authorization and Accounting (AAA) framework for applications such as network access or IP mobility.
Generally, for properly establishing and handling a communication connection between network elements such as a user equipment and another communication equipment or user equipment, a database, a server, etc., one or more intermediate network elements such as control network elements, support nodes, service nodes and interworking elements are involved which may belong to different communication networks.
Session border controllers (SBCs), for instance an interconnection border control functions (IBCF) or SIP application level gateway (SIP-ALG) within a proxy call state control function (P-CSCF), are frequently deployed at network borders between IMS networks and towards access networks or enterprise networks attached to an IMS. Such SBC frequently insert gateways into the user plane path, for instance a transition gateway (TrGW) or boarder gateway (BGW), for various purposes such as IP address and port translation and network protection. As an unfortunate side effect, the user plane is forced to traverse the same networks as the signalling plane even if much shorter user plane paths would be otherwise possible, for instance if caller and callee are located in the same visited or enterprise network, but the signalling involved still needs to traverse their home IMS networks. The purpose of optimised media routeing (OMR) is to remove unnecessary gateways from the user plane path.